H2AX Haploinsufficiency Modifies Genomic Stability and Tumor Susceptibility

Abstract

Histone H2AX becomes phosphorylated in chromatin domains flanking sites of DNA double-strand breakage associated with gamma-irradiation, meiotic recombination, DNA replication, and antigen receptor rearrangements. Here, we show that loss of a single H2AX allele compromises genomic integrity and enhances the susceptibility to cancer in the absence of p53. In comparison with heterozygotes, tumors arise earlier in the H2AX homozygous null background, and H2AX(-/-) p53(-/-) lymphomas harbor an increased frequency of clonal nonreciprocal translocations and amplifications. These include complex rearrangements that juxtapose the c-myc oncogene to antigen receptor loci. Restoration of the H2AX null allele with wild-type H2AX restores genomic stability and radiation resistance, but this effect is abolished by substitution of the conserved serine phosphorylation sites in H2AX with alanine or glutamic acid residues. Our results establish H2AX as genomic caretaker that requires the function of both gene alleles for optimal protection against tumorigenesis.

DNA double-strand break repair and development

“…A major function of g-H2AX is recruitment of DNA damage response factors to the sites of DNA DSBs to enhance the fidelity of DNA repair Fernandez-Capetillo et al, 2004). While H2AX is not strictly required for development in the mouse, its loss predisposes to genomic instability (Celeste et al, 2002(Celeste et al, , 2003. H2AX phosphorylation results in recruitment of mediator of DNA damage checkpoint protein 1 to the DNA break (Goldberg et al, 2003;Lou et al, 2003;Stewart et al, 2003), via binding to the g-H2AX C terminus, and this interaction is required for signal amplification and effective modulation of downstream ATM signaling (Bekker-Jensen et al, 2005;Stucki et al, 2005;Lou et al, 2006;Stucki and Jackson, 2006).…”

ERK activity facilitates activation of the S-phase DNA damage checkpoint by modulating ATR function

“…gH2AX foci also form at sites of physiological DSBs in lymphocytes and germ cells (Chen et al, 2000;Mahadevaiah et al, 2001;Petersen et al, 2001;Fernandez-Capetillo et al, 2003). Knocking out H2AX produces mice with immune deficiency and male infertility , while loss or reduction of H2AX compromises genome stability and facilitates tumorigenesis Bassing et al, 2003;Celeste et al, 2003). Taken together, gH2AX foci play an essential role in the cellular DNA damage response.…”

Breaking down cell cycle checkpoints and DNA repair during antigen receptor gene assembly

“…Ku70, DNA-PKcs, Artemis) or factors that survey V(D)J DSB intermediates (that is H2AX, 53BP1, Nbs1) leads to the rapid development of lymphomas (Bassing et al, 2003;Celeste et al, 2003;Difilippantonio et al, 2005;Ward et al, 2005;Morales et al, 2006). Thus, we should consider that different cell types and/or cell cycle phases may have distinct thresholds for the DSB checkpoint, with the activation threshold in lymphocytes being exquisitely sensitive to a single DSB generated in the G 0 /G 1 phase of the cell cycle.…”

“…Deficiency in Nbs1, H2AX and 53BP1 also increases the frequency of TCRa translocations (Celeste et al, 2003;Difilippantonio et al, 2005;Ward et al, 2005;Morales et al, 2006). However, the defect is less severe than in ATM À/À mice, and Nbs1, H2AX and 53BP1-deficient mice are not highly prone to spontaneous lymphomas (Bassing et al, 2003;Celeste et al, 2003;Difilippantonio et al, 2005;Ward et al, 2005;Morales et al, 2006).…”

“…However, the defect is less severe than in ATM À/À mice, and Nbs1, H2AX and 53BP1-deficient mice are not highly prone to spontaneous lymphomas (Bassing et al, 2003;Celeste et al, 2003;Difilippantonio et al, 2005;Ward et al, 2005;Morales et al, 2006). Moreover, in contrast to ATM knockouts, no impairment in the joining of V(D)J recombination associated breaks have been detected in H2AX À/À and 53BP1 À/À mice (Bassing et al, 2002;Manis et al, 2004;Ward et al, 2004).…”